Chloride electrode composed of ubiquitous elements for high-energy-density all-solid-state sodium-ion batteries

Inexpensive and safe energy-storage batteries with high energy densities are in high demand (e.g., for electric vehicles and grid-level renewable energy storage). This study focused on using NaFeCl4, comprising ubiquitous elements, as an electrode material for all-solid-state sodium-ion batteries. Monoclinic NaFeCl4, expected to be the most resource-attractive Fe redox material, is also thermodynamically stable. The Fe2+/3+ redox reaction of the monoclinic NaFeCl4 electrode has a higher potential (3.45 V vs. Na/Na+) than conventional oxide electrodes (e.g., Fe2O3 with 1.5 V vs. Na/Na+) because of the noble properties of chlorine. Additionally, NaFeCl4 exhibits unusually high deformability (99% of the relative density of the pellet) upon uniaxial pressing (382 MPa) at 298 K. NaFeCl4 operates at 333 K in an electrode system containing no electrolyte, thereby realizing next-generation all-solid-state batteries with high safety. A high energy density per positive electrode of 281 Wh kg−1 was achieved using only a simple powder press.

electrodes is suppressed when they are used in combination with solid electrolytes; thus, chloride electrodes are potentially suitable for use as high-voltage cathode materials for rechargeable ASSBs.
In this study, we focused on NaFeCl 4 as an electrode material for ASSBs, because NaFeCl 4 contains sodium chloride and the ubiquitous element Fe as the transition metal in the redox center.NaFeCl 4 is registered on the ISCD (#16994) 25 and has an orthorhombic crystal structure (S.G.: P2 1 2 1 2 1 ), with Fe in the center of the tetrahedron formed by the chloride ions.In electrode materials based on the Fe 2+/3+ redox reaction, Na 2 Fe 2 (SO 4 ) 3   26   exhibited a higher potential (3.8 V vs. Na/Na + ) than conventional oxide electrodes (Fe 2 O 3 27 , 1.5 V vs. Na/ Na + ) because of the noble properties of the SO 4 2− unit (which has an inductive effect).The same effect that was induced by the SO 4  2− ion was expected for the lighter Cl − ion (molar mass per charge Cl − : 34.5 g mol −1 , (SO 4 2− )/2:48 g mol −1 ), and the charge-discharge properties of the Fe 2+/3+ redox pair were evaluated at the NaFeCl 4 electrode.The theoretical capacity of this material is 121 mAh g −1 at one Na per NaFeCl 4 formula unit.We also assembled an ASSB using an electrode without electrolyte added to the electrode composite (hereafter referred to as an "electrolyte-free electrode").Conventional oxide electrodes exhibit low deformability and can be fabricated by adding soft sulfide electrolytes or other materials.However, the addition of electrolytes has the effect of decreasing the theoretical energy density per electrode composite, in which case the reaction distribution becomes more complicated.For chloride electrodes, the active material in the electrode may have high deformability.Therefore, in ASSBs, the electrode composites that do not contain solid electrolyte powders may surpass conventional battery systems in terms of their energy density.

Structure and deformability
The X-ray diffraction (XRD) pattern of the synthesized NaFeCl 4 sample (Fig. 1a) indicates a single phase consisting of monoclinic NaFeCl 4 .The peaks of the raw material are no longer visible.The relative density of the uniaxially compressed pellet was calculated from the apparent density of the compact and crystal lattice density of the monoclinic NaFeCl 4 (2.31 g cm −3 ).The value of 99.1% for NaFeCl 4 (at 382 MPa) is higher than those for Li 2 FeCl 4 (92% at 382 MPa) 28 and Li 3 TiCl 6 (86.1% at 350 MPa) 29 , which have recently been reported as highly deformable electrodes.The cross-sectional scanning electron microscopy (SEM) image of the NaFeCl 4 powder compact (Fig. 1b) indicates that the grains were crushed by compaction, resulting in a dense structure with illdefined grain boundaries.These results indicate that the NaFeCl 4 powder has high deformability.) and sulfide (Na x TiS 2

Electrochemical performance
An ASSB with a solid electrolyte was fabricated to evaluate this strongly ionic electrode-active material without it leaching into the electrolyte.In addition, taking advantage of this deformability, electrolyte-free electrodes were fabricated for application in ASSBs, and their charge-discharge characteristics were evaluated.Based on the conductivity diffusion coefficient (Fig. 1c) obtained from the impedance plot (Supplementary Fig. 1), the battery operating temperature was set at 333 K to ensure that the diffusion coefficient is higher than those of conventional electrode materials.The solid electrolyte (Na  and S3).The resulting reversible capacity was 90.8 mAh (g-NaFeCl 4 ) −1 (81.7 mAh (g-positive electrode) −1 ), and the average working potential was ~ 3.45 V (vs.Na/Na + ), as is evident from the constant-current charge-discharge curves (Fig. 2a) and the dQ/dV curves (Figure S4).The results of the impedance measurements (Fig. 2b) indicate only a small semicircular resistance and no significant increase in the first charge-discharge process or after the cycle test.The battery also exhibits relatively stable cycling characteristics over 10 cycles (Fig. 2c).Based on the reversible capacity of this discharge capacity (~ 90 mAh g −1 ), the gravimetric energy density per positive electrode was calculated to be 281 Wh kg −1 at the reference potential of Na (~ 3.45 V).In Table 1, this value is compared with previously reported energy densities of bulk ASSBs with high-potential operation (> 3 V).This shows that the ASSB fabricated in this study via a simple process using only pressed powders, which does not require any coating or sintering process on the -Z'' / : After 1 st discharge After 1 st charge After cycle test  , of the ASSB in this study with that of other bulk ASSB with a high-potential cathode (> 3 V).a E m is the energy density per positive electrode weight assuming a Na metal anode.If not stated otherwise, it was calculated from the initial discharge curve.Other information, such as the materials used, their mixing ratios, fabrication conditions, and working temperatures, is also shown.www.nature.com/scientificreports/surface of the cathode active material, has higher energy density than other reported bulk ASSBs using inorganic and/or polymer electrolytes.The redox mechanism of the NaFeCl 4 electrode was investigated by X-ray photoelectron spectroscopy (XPS) before and after the charge-discharge process.The Fe 2p XPS profile (Fig. 3) consists of two sets of doublet peaks (Fe 2p 3/2 and Fe 2p 1/2 ) and their satellite peaks.Deconvolution of each spectrum using the pseudo-Voigt function revealed that the Fe 2p 2/3 peak is located near 711.0 eV before and after charge, whereas a high-intensity peak appears near 710.5 eV, and the intensity of the peak at 711.0 eV is lower after discharge.The Fe 2p 2/3 peaks of FeCl 2 and FeCl 3 in the reference sample appear at 710.6 eV and 711.3 eV, respectively 42,43 , with the low-and high-energy peaks attributable to Fe 2+ and Fe 3+ , respectively.The peak ratio after discharge was approximately 3:1, which is consistent with the fact that the discharge capacity was approximately 75% of the theoretical capacity (121.5 mAh g −1 ).This indicates that the charge-discharge process proceeded via the redox reaction of Fe 2+/3+ in NaFeCl 4 .As mentioned previously, the redox reaction of Fe 2+/3+ has been reported to have a low potential of approximately 1.5 V in conventional oxides (Fe 2 O 3 ).In this material, the inductive effect of chlorine may be responsible for the higher potential (3.45 V vs. Na/Na + ), which would be responsible for the high energy density listed in Table 1.The XPS profile after charging revealed a reversible return to the original Fe 3+ state before the charge-discharge process.The XRD patterns before and after charging (Supplementary Fig. S4) also show a reversion to monoclinic NaFeCl 4 , indicating the occurrence of a reversible charge-discharge reaction involving a Fe 2+/3+ redox reaction.The synthesis of Na 2 FeCl 4 , which can be charge-started, has not yet been reported; therefore, it is expected to be evaluated in future studies.
In summary, the NaFeCl 4 electrode, composed of ubiquitous elements, was evaluated for application in a low-cost storage battery with high energy density and safety.An ASSB was operated at 333 K with an electrolytefree electrode owing to the high deformability derived from chloride ions (relative density = 99% of the pellet uniaxially compressed at 298 K).In addition, owing to the inductive effect of chloride, high-potential operation (3.45 V vs. Na/Na + ) was demonstrated with the most attractive Fe redox reaction (Fe 2+/3+ ) in terms of the elemental strategy.Consequently, an outstanding energy density (281 Wh (kg-positive electrode) −1 ) was achieved for conventional bulk all-solid-state sodium-ion batteries without sintering or electrode coating treatment.This study demonstrates the potential of NaCl-based materials as high-energy-density electrode materials, which have previously been difficult to evaluate because of their elution into the electrolyte.

Methods
Preparation and evaluation of all-solid-state cells using NaFeCl 4 electrodes NaFeCl 4 was synthesized from NaCl (Wako Pure Chemical Industries, Ltd., 99.5%) and FeCl 3 (Sigma-Aldrich Japan LLC, 99.9%) powders by a mechanochemical method 28 .A stoichiometric mixture was placed in a 45 mL www.nature.com/scientificreports/stainless steel pot with 74 ZrO 2 balls (diameter = 5 mm) and milled using a planetary ball mill apparatus (Fritsch Japan Co., Ltd., P-7 classic-line, Japan) at a rotation speed of 300 rpm for 5 h.The as-produced yellow sample was probed by XRD (MiniFlex 600, Rigaku, Japan, CoKα line).The conductivity diffusion coefficient was measured at 298 and 333 K using the AC impedance method (VSP Potentiostat, BioLogic, France) with an AC voltage of 300 mV and a measurement frequency range of 10 2 -10 6 Hz.Pellets (diameter = 10 mm, thickness = ~ 0.50 mm) were prepared by sandwiching the powder between stainless steel plates and compressing under 382 MPa at 298 K.The pellets contain 10 wt% of Ketjen black (KB) as a conductivity aid.The cross-section of a pellet was polished with a #2000 file, and cross-sectional images were acquired using SEM (JSM-6360LV, JEOL, Japan) at a voltage of 10 kV.All the procedures were performed under dry Ar gas.Electrolyte-free electrodes were prepared with NaFeCl 4 , and the charge-discharge characteristics of the allsolid-state sodium-ion batteries were evaluated.The NaFeCl 4 electrode was mixed with KB (mixing ratio of NaFeCl 4 :KB = 90:10 wt%) as a conductive aid via ball milling at 300 rpm for 1 h.The solid electrolyte was the sulfide Na 3 PS 4 44 and/or chloride Na 2.25 Y 0.25 Zr 0.75 Cl 6 45 , and the negative electrode (counter electrode) was Na 10 Sn 4 -AB (AB: acetylene black; mixing ratio of Na 10 Sn 4 :AB = 90:10 wt%) 46 .For cell fabrication, approximately 60 mg of the electrolyte was first placed in a polycarbonate pressure vessel with a cylindrical inner diameter of 10 mm, sandwiched between two pieces of stainless steel, and pressurized to 96 MPa.Subsequently, the cathode material was placed on one side and pressed at 96 MPa, whereas the anode material was placed on the other side and pressed at 382 MPa.The cells were then screwed in from the top and bottom and restrained.When two layers of the electrolyte were used, approximately 30 mg of the Na 3 PS 4 electrolyte was used on the anode side and approximately 30 mg of the Na 2.25 Y 0.25 Zr 0.75 Cl 6 electrolyte on the cathode side.Charging and discharging were evaluated using a potentiostat/galvanostat (VSP Potentiostat, BioLogic, France) at a temperature of 333 K, current density of 6.4 μA cm −2 , and starting from the discharging process (Na storage).AC impedance measurements were performed in the frequency range of 1 − 10 6 Hz after charging and discharging.The charge-discharge mechanism of NaFeCl 4 was investigated by conducting Fe 2p XPS measurements on the electrode before and after charging and discharging using a Cr K α radiation source (PHI Quantes, ULVAC-PHI, Inc., USA) without surface etching treatment, which would be a concern in terms of alteration.

Figure 1 . 3 ,and NaCoO 2 32
Figure 1.Characterization of synthesized NaFeCl 4 .(a) XRD patterns of NaFeCl 4 after milling and the raw material and the simulated pattern of orthorhombic NaFeCl 4 from the ICSD database (#16994).(b) Crosssectional SEM image of the pellet uniaxially compressed.(c) Conductivity diffusion coefficient of NaFeCl 4 obtained from AC impedance measurements at 298 and 333 K and the chemical diffusion coefficients of typical oxide (Na x MnO 3 ,and NaCoO 2 32

Figure 3 .
Figure 3. Fe 2p XPS results of the NaFeCl 4 electrode before and after the charge-discharge process.The spectra were fitted with 2p 3/2 and 2p 1/2 of Fe 2+/3+ and satellite peaks.
3 PS 4 |Na 2.25 Y 0.25 Zr 0.75 Cl 6 ) consists of two layers, one on the anode (Na 10 Sn 4 ) side and the other on the cathode (NaFeCl 4 ) side, to suppress the reactions between the electrodes and electrolytes (cell configuration: Na 10 Sn 4 + acetylene black (AB)|Na 3 PS 4 |Na 2.25 Y 0.25 Zr 0.75 Cl 6 |NaF eCl 4 + KB) based on an examination of reactions like the oxidation of Na 3 PS 4 , as shown in Supporting Section 1 (Supplementary Figs.S2

Table 1 .
Comparison of the energy density per positive electrode weight, E m